1. Field of the Invention
The present invention relates to a modulation apparatus and more particularly to a universal modulator architecture for generating various band efficient signals, such as 8 PSK, 16 PSK, 16 QAM, 32 QAM, 64 QAM and 12/4 QAM.
2. Description of the Prior Art
Bandwidth efficiency is important in digital communication systems. It is the ratio of the rate of information transmission to the bandwidth required for the system to operate properly. The increasing demand for information transmission and limited resource of bandwidth allocations has focused interest on communications systems offering a high level of bandwidth efficiency.
Digital communication systems typically operate by sending symbols between a transmitter and a receiver selected from a predetermined alphabet of symbols, where each symbol represents some number of information bits. The required bandwidth is related to the rate of symbol transmission is typically expressed in terms of number of symbols per second. A strategy for increasing the bandwidth efficiency of a digital communication system is to use a larger alphabet of symbols where each symbol represents more information bits.
For example, in a simple biphase shift keyed system, an alphabet of two symbols is used. Each symbol represents a single bit (1 or 0). The phase of a constant amplitude carrier signal is shifted by 0° or 180° to represent each of the two symbols. Each symbol is represented by a dot as shown in FIG. 1a. 
Increasing the alphabet of transmitted symbols to four allows two bits of digital information to be transmitted on each symbol. This is referred to as quadraphase shift keying (QPSK). The carrier is transmitted at one of four possible phases, separated by 90°, to represent each symbol as illustrated in FIG. 1b. Such QPSK modulation techniques are known in the art and described in U.S. Pat. Nos. 5,440,259; 5,615,230; 5,440,268; 5,550,868; 5,598,441; 5,500,876 and 5,485,489, hereby incorporated by reference.
This may be extended to 8 PSK and 16 PSK, where an alphabet of eight or sixteen symbols represent three or four bits of digital information. The carrier is then transmitted at one of eight or sixteen phases separated by 45° or 22.5°, for example as shown in FIGS. 1c and 1d. Note that in all the above cases the amplitude (distance to the origin on the plot) is constant for all symbols.
The reduced angular separation of transmitted symbols increases the likelihood of erroneous symbol decisions in the receiver resulting in information bit transmission errors. This limits the usefulness of larger alphabets of PSK symbols.
By varying the amplitude in addition to the phase of a carrier, the communication system may operate with a larger symbol alphabet while preserving the separation of symbols in the receiver. A modulation format that exploits this is known as quadrature amplitude modulation (QAM) is illustrated in FIGS. 1e–1g. 16 QAM, 32 QAM and 64 QAM transmit four, five and six bits of information with each symbol respectively. Note that multiple amplitudes and phases differentiate the symbols. Such QAM modulation techniques are relatively well known in the art. Example of such QAM modulation techniques are discussed in U.S. Pat. Nos. 5,612,651; 5,343,499; 5,363,408 and 5,307,377, hereby incorporated by reference.
FIG. 1h illustrates 12/4 QAM which is a special case of 16 QAM. Only two values of amplitude are used for 12/4 QAM as compared with three for 16 QAM. The lower amplitude symbols are separated by 90° and the larger amplitude symbols are separated by 30°. 12/4 QAM is a desirable alternative to 16 QAM because it makes more efficient use of limited transmitter power.
The modulators for generating bandwidth efficient signals, such as QPSK and QAM signals are relatively complex. Serial modulators are known for generating PSK waveforms. However, such serial modulators do not have the capability for amplitude modulation and thus cannot be used in QAM applications. Parallel modulators are also known that are adapted to provide both phase and amplitude modulation, making such modulators suitable for both QPSK and QAM applications. However, the complexity and critical alignment of such parallel modulators make such modulators relatively expensive. A digital vector modulator is also known which overcomes many of the problems mentioned above. However, such digital vector modulators are only suitable for use at lower data rate applications.
In order to obviate the need for different modulators for different modulating techniques, a universal architecture for a modulator is disclosed in U.S. Pat. No. 5,648,985. In particular, the '985 patent discloses a universal architecture for generating both QAM and PSK signals. Unfortunately, such a universal modulator is not suitable and cannot be used for other band efficient modulation techniques, such as FSK.